Question about chicks, outside, grit, and rocks......agh!!

My chicks are about 3 weeks old and I want them to be able to play outside for a few. We live in Southern Arizona so I am not too worried about the temp. I do have a verrrrryyy rocky yard. Not the rock you buy but the God given natural kind...lol. I am worried the chicks will swallow something they aren't supposed too....am I being silly goose or what? Thanks for the help: